The present invention relates generally to attitude or angular velocity or sensor alignment estimate adjustment for a vehicle, and more particularly, to algorithms involving attitude or angular velocity or sensor alignment determination, using star position measurements and using multiple exclusion zones.
Satellites and other vehicles are in widespread use for various purposes including scientific research and communications. Many scientific and communications missions, however, cannot be accurately fulfilled without consistently monitoring and controlling the 3-axis attitude and angular velocity of the vehicle. In many applications, the vehicle must be oriented to transmit signals in particular directions or to receive signals from specifically located sources. Furthermore, in such a situation, the vehicle angular velocity must be such so as to maintain the desired orientation, over time. Without accurate control over vehicle 3-axis attitude and angular velocity, the transmission or reception of such signals is hindered and at times impossible.
Such control requires systems for 3-axis attitude and angular velocity determination, which generally include one or more star trackers and a 3-axis gyroscope. During normal operation, star trackers or star sensors provide attitude-related information and the 3-axis gyroscope is needed to provide angular velocity information. As there are inherent, and time-varying, errors from star trackers, star sensors, and gyros, it is often necessary to constantly estimate such errors, in order to compensate for them. One common method of doing so is to correlate star tracker or sensor position measurements of stars with known positions of the same stars, as listed in a star catalog, or database. Discrepancies between the measured and predicted positions allow direct estimation of tracker error, and indirect estimation of gyro error. Knowing such errors allows estimation of attitude or angular velocity, or refinement of existing estimates. Furthermore, if there are multiple star trackers or star sensors on-board, such correlations allow determination of the alignment of such trackers or sensors, with respect to each other; such determination yields greater accuracy in future attitude and angular velocity estimates.
Stellar Inertial Attitude Determination (SIAD) algorithms employ a carefully designed star catalog or database for selection and identification of stars tracked by star trackers or star sensors. The current known art in star selection for star trackers mainly concentrates on generating star catalogs with certain properties. Typically, one method of locating entries in a star catalog that correspond to stars in a tracker field of view (FOV) uses a standard binary search that is sorted by declination only. This method is not very efficient because it involves searching through hundreds of entries to find the stars that are located within the FOV.
Various other methods involve generation of multiple overlapping sub-catalogs that contain stars for a specific FOV in the sky. At any point in time, the stars within a star tracker's FOV will reside in one or more of these sub-catalogs. Each star catalog entry that is represented in a FOV sub-catalog has been determined, using on-board processing, to be in the tracker FOV.
In a SIAD algorithm, the entries corresponding to stars intruded by bright objects such as planets, asteroids, or comets, need to be excluded from the star catalog or sub-catalog, i.e., excluded from consideration by the algorithm. This is so that bright objects are not mistaken for stars, or the light from the objects does not corrupt the measurements made by the star sensor. Traditional object-based catalog entry exclusion is performed on the ground. These stars intruded by planets or other bright objects are excluded from a revised copy of the on-board star catalog. The revised star catalog is then uploaded to the spacecraft control processor (SCP). The uploading is time-consuming and may easily be interrupted. This may cause delays in the proper attitude determination or errors in the on-board version of the catalog.
Traditionally, there is only one stayout zone for each object in the exclusion algorithm. All the stars inside the stayout zone are excluded from the star catalog so that the bright planet/object will not be mistaken as a star during attitude determination. The exclusion zones may be excessively large and over-inclusive. The number of stars excluded may be such that attitude determination performance may be reduced.
It would therefore be desirable to provide a system that decreases the number of excluded stars to improve the performance of the attitude determination system.